Process for producing 2,4′-dihydroxydiphenylsulfone

ABSTRACT

A process for producing 2,4′-dihydroxydiphenylsulfone which comprises separating 4,4′-dihydroxydiphenylsulfone by crystallization from a mixture containing 4,4′-dihydroxydiphenylsulfone, 2,4′-dihydroxydiphenylsulfone, phenolsulfonic acid and phenol which is obtained by dehydration of phenol and sulfuric acid or phenolsulfonic acid to obtain a mixture having a content of 2,4′-dihydroxydiphenylsulfone greater than the content of 4,4′-dihydroxydiphenylsulfone, crystallizing 2,4′-dihydroxydiphenylsulfone by adjusting the composition of the solvent of the obtained mixture so that the ratio of the amounts by weight of phenol to water is 10:90 to 90:10, and separating 2,4′-dihydroxydiphenylsulfone by filtration; and a process as described above in which the filtrate obtained by the filtration is used as the raw material for the hydration.

This application is the United States national phase application ofInternational Application PCT/JP03/00571 filed Jan. 22, 2003.

TECHNICAL FIELD

The present invention relates to a process for producing2,4′-dihydroxydiphenylsulfone. More particularly, the present inventionrelates to a process for producing 2,4′-dihydroxydiphenylsulfone whichcomprises efficiently separating 2,4′-dihydroxydiphenylsulfone from amixture comprising 4,4′-dihydroxydiphenylsulfone and2,4′-dihydroxydiphenylsulfone and effectively utilizing phenolsulfonicacid contained in the reaction mixture as the raw material.

BACKGROUND ART

A mixture of 4,4′-dihydroxydiphenylsulfone and2,4′-dihydroxydiphenylsulfone can be obtained by condensation withdehydration of phenol and sulfuric acid. 4,4′-Dihydroxydiphenylsulfoneis known as the raw material for polyether sulfone which is anengineering plastic exhibiting excellent heat resistance and a monomerfor improving heat resistance of polycarbonates.2,4′-Dihydroxydiphenylsulfone is a useful compound which is used as adeveloper for heat-sensitive recording materials and, in particular,provides excellent heat-sensitive recording paper exhibiting excellentcoloring property, causing little fog on the background and stored withstability.

The product of condensation with dehydration of phenol and sulfuric acidis a mixture of 4,4′-dihydroxydiphenylsulfone and2,4′-dihydroxydiphenylsulfone. Therefore, it is necessary for obtaininghighly pure 4,4′-dihydroxydiphenylsulfone or2,4′-dihydroxydiphenylsulfone that these compounds are separated fromeach other. Various processes for the separation have been examined.

As for the process for separating 4,4′-dihydroxydiphenylsulfone, forexample, a process in which a mixture of the isomers ofdihydroxydiphenylsulfone is dissolved into phenol by heating,4,4′-dihydroxydiphenylsulfone alone is separated by crystallization asan addition compound with phenol by cooling, and then crystals of4,4′-dihydroxydiphenylsulfone are obtained by a treatment of heating, isproposed in Japanese Patent Application Laid-Open No. Showa57(1982)-77667 as the process for separating a highly pure4,4′-dihydroxydiphenylsulfone from a mixture of the isomers. In JapanesePatent Application Laid-Open No. Showa 50(1975)-106936, a process inwhich a product of the reaction between phenol and sulfuric acid isbrought into contact with an aqueous solution of phenol having aconcentration of 3 to 35% by weight, and 4,4′-dihydroxy-diphenylsulfoneis separated by crystallization, is proposed as the process forproducing 4,4′-dihydroxydiphenylsulfone having a small content of theisomer. However, in accordance with these processes, it is difficultthat 2,4′-dihydroxydiphenylsulfone is separated from the residualmixture obtained after the separation of 4,4′-dihydroxydiphenylsulfone.

In Japanese Patent Application Laid-Open No. Heisei 10(1998)-25277, aprocess in which phenol and a sulfonating agent are brought intoreaction in a solvent of o-dichlorobenzene, the amount of the unreactedphenol in the reaction mixture is adjusted to 2 to 20% by weight basedon the total of the amounts of the unreacted phenol ando-dichlorobenzene, and the total of the amounts of the unreacted phenoland o-dichlorobenzene is adjusted to 2 to 7 times the theoretical yieldof dihydroxydiphenylsulfones when the reaction is completed so that4,4′-dihydroxydiphenylsulfone is separated by crystallization from thereaction mixture, and 2,4′-dihydroxydiphenylsulfone is isolated form thefiltrate, is proposed as the process for producing a combination ofhighly pure 2,4′-dihydroxydiphenylsulfone and4,4′-dihydroxydiphenylsulfone industrially advantageously. However,since this process uses the chlorine-based organic solvent, this processrequires a facility for recovery of the solvent and, moreover, tends tocause problems on the environment.

In Japanese Patent Application Laid-Open No. Heisei 9(1997)-40635, aprocess in which, in an aqueous solution of a mixture of2,4′-dihydroxydiphenylsulfone and 4,4′-dihydroxydiphenylsulfone,2,4′-dihydroxydiphenylsulfone is dissolved as a dialkali metal salt and4,4′-dihydroxydiphenylsulfone is separated as a monoalkali metal salt inthe presence of a hydroxide of an alkali metal, is proposed as theprocess for efficiently producing highly pure2,4′-dihydroxydiphenylsulfone. However, in accordance with this process,great amounts of sodium hydroxide and sulfuric acid are used for theseparation, and the efficiency is small from the standpoint of the cost.

The present invention has an object of providing a process forefficiently and economically advantageously separating2,4′-dihydroxydiphenylsulfone from a reaction mixture comprising4,4′-dihydroxydiphenylsulfone and 2,4′-dihydroxydiphenylsulfone andeffectively utilizing phenolsulfonic acid contained in the reactionmixture as the raw material.

DISCLOSURE OF THE INVENTION

As the result of intensive studies by the present inventors to overcomethe above problems, it was found that, when a mixture of the isomers hada content of 2,4′-dihydroxydiphenylsulfone greater than the content of4,4′-dihydroxydiphenylsulfone, 2,4′-dihydroxydiphenylsulfone wasselectively separated by crystallization from a solvent containingphenol and water in amounts such that the ratio of the amounts by weightof phenol to water was in the range of 10:90 to 90:10, and the filtrateobtained after separation of the crystals by filtration could beeffectively utilized as the raw material for producingdihydroxydiphenylsulfones. The present invention has been completedbased on this knowledge.

The present invention provides:

-   (1) A process for producing 2,4′-dihydroxydiphenylsulfone which    comprises separating 4,4′-dihydroxydiphenylsulfone by    crystallization from a mixture comprising    4,4′-dihydroxydiphenylsulfone, 2,4′-dihydroxydiphenylsulfone,    phenolsulfonic acid and phenol which is obtained by dehydration of    phenol and sulfuric acid or phenolsulfonic acid so that a mixture    having a content of 2,4′-dihydroxydiphenylsulfone greater than a    content of 4,4′-dihydroxydiphenylsulfone is obtained, crystallizing    2,4′-dihydroxydiphenylsulfone by adjusting a composition of a    solvent of an obtained mixture so that a ratio of an amount by    weight of phenol to an amount by weight of water is in a range of    10:90 to 90:10, and separating 2,4′-dihydroxydiphenylsulfone by    filtration; and-   (2) A process for producing 2,4′-dihydroxydiphenylsulfone described    in (1), which comprises adding phenol and sulfuric acid to a    filtrate obtained by the separation of 2,4′-dihydroxydiphenylsulfone    by filtration, conducting dehydration using a resultant mixture,    separating 4,4′-dihydroxydiphenylsulfone by crystallization from a    product of dehydration comprising 4,4′-dihydroxydiphenylsulfone,    2,4′-dihydroxydiphenylsulfone, phenolsulfonic acid and phenol so    that a mixture having a content of 2,4′-dihydroxydiphenylsulfone    greater than a content of 4,4′-dihydroxydiphenylsulfone is obtained,    crystallizing 2,4′-dihydroxydiphenylsulfone by adjusting a    composition of a solvent of an obtained mixture so that a ratio of    an amount by weight of phenol to an amount by weight of water is in    a range of 10:90 to 90:10, and separating    2,4′-dihydroxydiphenylsulfone by filtration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram exhibiting a solubility curve of a mixture ofisomers of dihydroxydiphenylsulfone.

FIG. 2 shows a diagram exhibiting a solubility curve of a mixture ofisomers of dihydroxydiphenylsulfone.

THE MOST PREFERRED EMBODIMENT TO CARRY OUT THE INVENTION

In the process for producing 2,4′-dihydroxydiphenylsulfone of thepresent invention, 4,4′-dihydroxydiphenylsulfone is separated bycrystallization from a mixture comprising 4,4′-dihydroxydiphenylsulfone,2,4′-dihydroxydiphenylsulfone, phenolsulfonic acid and phenol which isobtained by dehydration of phenol and sulfuric acid or phenolsulfonicacid, and a mixture having a content of 2,4′-dihydroxydiphenylsulfonegreater than the content of 4,4′-dihydroxydiphenylsulfone is obtained.2,4′-Dihydroxydiphenylsulfone is crystallized by adjusting thecomposition of the solvent of the obtained mixture so that the ratio ofthe amount by weight of phenol to the amount by weight of water is inthe range of 10:90 to 90:10, and the crystals of2,4′-dihydroxydiphenylsulfone are separated by filtration.

A reaction mixture comprising 4,4′-dihydroxydiphenylsulfone and2,4′-dihydroxydiphenylsulfone can be obtained by mixing phenol andsulfuric acid and/or phenolsulfonic acid, followed by heating theresultant mixture and removing formed water by distillation as anazeotrope. It is preferable that the reaction is conducted under areduced pressure. It is preferable that, where necessary, phenol isadded during the reaction to supplement phenol removed to the outside ofthe system by the azeotropic distillation with water. In general,4,4′-dihydroxydiphenylsulfone is formed in a greater amount than2,4′-dihydroxydiphenylsulfone.

In the reaction of phenol and sulfuric acid, a phosphorus compound suchas phosphonic acid, phosphinic acid and phosphoric acid may be present.When the phosphorus compound is present, the content of2,4′-dihydroxydiphenylsulfone in the formed mixture of the isomers ofdihydroxydiphenylsulfone can be increased. In general, the reactionmixture obtained by the reaction of phenol and sulfuric acid containsphenolsulfonic acid in an amount corresponding to an yield of 10 to 30%based on the amount of sulfuric acid.

As for the solubility of a mixture of the isomers ofdihydroxydiphenylsulfone having a content of4,4′-dihydroxydiphenylsulfone greater than the content of2,4′-dihydroxydiphenylsulfone in a mixed solvent of phenol and water orin phenol, the solubility of 4,4′-dihydroxydiphenylsulfone is smallerthan the solubility of 2,4′-dihydroxydiphenylsulfone when the ratio ofthe amount by weight of phenol to the amount by weight of water is 70:30or greater. FIG. 1 shows a diagram exhibiting the relation between thetemperature and the solubilities of the isomers in phenol when4,4′-dihydroxydiphenylsulfone and 2,4′-dihydroxydiphenylsulfone arepresent in amounts such that the ratio of the amounts by weight is50:50. Therefore, 4,4′-dihydroxydiphenylsulfone can be selectivelyseparated by the crystallization treatment of the reaction mixtureobtained by the dehydration of phenol and sulfuric acid and/orphenolsulfonic acid using a mixed solvent containing phenol and water inamounts such that the ratio of the amounts is 70:30 or greater or usingphenol.

Since almost no water is contained in the reaction mixture, it ispreferable that 4,4′-dihydroxydiphenylsulfone is separated bycrystallization by adding phenol to the reaction mixture. The amount ofthe crystallized 4,4′-dihydroxydiphenylsulfone to provide the content of2,4′-dihydroxydiphenylsulfone greater than the content of4,4′-dihydroxydiphenylsulfone in the filtrate obtained by thecrystallization of 4,4′-dihydroxydiphenylsulfone, followed by thefiltration, can be calculated, and the amount of the added phenol can beobtained based on the solubility using the amount obtained by thecalculation. The filtrate having a content of2,4′-dihydroxydiphenylsulfone greater than the content of4,4′-dihydroxydiphenylsulfone can be obtained as follows: phenol isadded to the reaction mixture; the resultant mixture is heated until theentire fluid becomes homogeneous; the heated fluid is cooled, andcrystals of 4,4′-dihydroxydiphenylsulfone are allowed to growsufficiently and then separated by filtration. Since the crystals of4,4′-dihydroxydiphenylsulfone separated by the filtration containsphenol and phenolsulfonic acid, it is preferable that the crystals arewashed with hot water and dried. The obtained4,4′-dihydroxydiphenylsulfone has a purity of 95% by weight or greater.

In the filtrate having a content of 2,4′-dihydroxydiphenylsulfonegreater than the content of 4,4′-dihydroxydiphenylsulfone which isobtained by the filtration, the solvent is adjusted to have acomposition such that the ratio of the amount by weight of phenol to theamount by weight of water is in the range of 10:90 to 90:10. FIG. 2shows a diagram exhibiting the relation between the composition of thesolvent and the solubilities of the isomers at 30° C. when4,4′-dihydroxydiphenylsulfone and 2,4′-dihydroxydiphenylsulfone arepresent in amounts such that the ratio of the amounts by weight is50:50. The composition and the amount of the solvent necessary forselectively crystallizing 2,4′-dihydroxydiphenylsulfone can be obtainedby calculation based on the relation between the composition of thesolvent and the solubilities of the isomers. It is preferable that theratio of the amounts by weight of phenol and water is selected so thatthe solvent has the composition giving a small solubility of2,4′-dihydroxydiphenylsulfone and a great difference between thesolubilities of 4,4′-dihydroxydiphenylsulfone and2,4′-dihydroxydiphenylsulfone. In general, the composition and theamount of the solvent can be adjusted by removing phenol contained inthe filtrate by distillation and adding water. It is preferable that theremoval of phenol from the filtrate by distillation is conducted under areduced pressure at a temperature of 120° C. or lower. When thetemperature exceeds 120° C., there is the possibility that thedehydration takes place between phenol and phenolsulfonic acid and4,4′-dihydroxydiphenylsulfone is formed.

After the composition and the amount of the solvent are adjusted byremoving phenol by distillation and adding water, the mixture is heateduntil the entire fluid becomes homogeneous, and the crystals of2,4′-dihydroxydiphenylsulfone are then selectively separated by cooling,followed by filtration. It is preferable that the temperature obtainedby the cooling is around the room temperature. When the temperature isexcessively low during the operation of the filtration, there is thepossibility that 4,4′-dihydroxydiphenylsulfone is crystallized todecrease the purity of crystals of 2,4′-dihydroxydiphenylsulfone, andthe filtration becomes difficult due to the muddy condition of themixture. It is preferable that the crystals separated by the filtrationare washed with water and dried since the crystals contain phenol andphenolsulfonic acid. The obtained 2,4′-dihydroxydiphenylsulfone has apurity of 85% by weight or greater.

In the process of the present invention, the filtrate obtained by thefiltration to separate 2,4′-dihydroxydiphenylsulfone is used as the rawmaterial for producing dihydroxydiphenylsulfone. Since the filtrateobtained by the filtration to separate 2,4′-dihydroxydiphenylsulfonecontains phenol, phenolsulfonic acid and water, phenolsulfonic acidwhich is difficult to be separated is used as the raw material, and theamount of the raw material used for producing dihydroxydiphenylsulfonecan be decreased. The filtrate may be used for the dehydration afterwater is removed by distillation and then phenol and sulfuric acid areadded. Alternately, phenol and sulfuric acid may be added to thefiltrate, and the resultant mixture is heated to remove water containedin the filtrate by distillation together with water formed by thedehydration.

The mixture containing 4,4′-dihydroxydiphenylsulfone,2,4′-dihydroxydiphenylsulfone, phenolsulfonic acid and phenol which isobtained by adding phenol and sulfuric acid to the filtrate formed byfiltration of 2,4′-dihydroxydiphenylsulfone, followed by the dehydrationof the resultant mixture, can be treated in the same manner as that inthe treatments of the mixture containing 4,4′-dihydroxydiphenylsulfone,2,4′-dihydroxydiphenylsulfone, phenolsulfonic acid and phenol which isobtained by the dehydration of phenol and sulfuric acid.4,4′-Dihydroxydiphenylsulfone is separated by crystallization to obtaina mixture having a content of 2,4′-dihydroxydiphenylsulfone greater thanthe content of 4,4′-dihydroxydiphenylsulfone.2,4′-Dihydroxydiphenylsulfone is crystallized by adjusting thecomposition of the mixture so that the ratio of the amounts by weight ofphenol to water is in the range of 10:90 to 90:10; and the formedcrystals are separated by filtration. The filtrate formed above by thefiltration and containing phenol and phenolsulfonic acid can be usedrepeatedly as the raw material for producing dihydroxydiphenylsulfone bythe dehydration of phenol and sulfuric acid and/or phenolsulfonic acid.

In accordance with the process of the present invention,4,4′-dihydroxydiphenylsulfone and 2,4′-dihydroxydiphenylsulfone whichare industrially useful can be separated from the reaction mixturewithout using specific solvents or chemicals, and the unreactedphenolsulfonic acid contained in the reaction mixture can be used as theraw material for the dehydration and effectively utilized. In theprocess of the present invention, since phenol which is one of thesolvents used for separation of 4,4′-dihydroxydiphenylsulfone and2,4′-dihydroxydiphenylsulfone is the raw material used for producingdihydroxydiphenylsulfone, and water which is the other solvent is abyproduct formed in the production of dihydroxydiphenylsulfone, theprocess of the present invention requires no additional apparatuses forrecovery or purification and can be conducted economically.

EXAMPLES

The present invention will be described more specifically with referenceto examples in the following. However, the present invention is notlimited to the examples.

The contents of 4,4′-dihydroxydiphenylsulfone,2,4′-dihydroxydiphenylsulfone, phenolsulfonic acid and phenol weredetermined in accordance with the high performance liquidchromatography.

Example 1

Into a reactor, 1,296 g of phenol, 529 g of sulfuric acid and 38 g ofphosphonic acid were placed, and the dehydration was conducted under areduced pressure of 74.6 to 34.7 kPa at 150 to 165° C. for 6 hours. Theamount of the distillate of a mixture composed of phenol and waterduring the reaction was 547 g.

Into the reactor, 267 g of phenol was added, and the dehydration wasconducted under a reduced pressure of 34.7 to 13.3 kPa for 2 hours. Theamount of the entire distillate was 675 g. Then, 267 g of phenol wasadded into the reactor and the dehydration was conducted under a reducedpressure of 34.7 to 13.3 kPa for 2 hours. The amount of the entiredistillate was 806 g. Then, 267 g of phenol was added into the reactor,and, after the dehydration was conducted under a reduced pressure of34.7 to 13.3 kPa for 2 hours, the reaction was completed. The amount ofthe entire distillate was 913 g.

The amount of the obtained reaction mixture was 1,751 g, and thecontents of the non-volatile components in the obtained reaction mixturewere as follows: 4,4′-dihydroxydiphenylsulfone: 44% by weight;2,4′-dihydroxydiphenylsulfone: 20% by weight; phenolsulfonic acid: 10%by weight; phenol: 24% by weight; and other components: 2% by weight.The yield of the dihydroxydiphenylsulfones based on the sulfuric acidused as the raw material was 82%.

To 1,751 g of the reaction mixture obtained above by the dehydration,683 g of phenol was added, and the resultant mixture was heated at 120°C. so that the entire fluid became homogeneous. The temperature of themixture in the homogeneous condition was slowly lowered to 60° C. Themixture was kept being stirred at 60° C. for 1 hour, and crystals of4,4′-dihydroxydiphenylsulfone were allowed to grow. The formed crystalswere separated by filtration, washed with water and dried, and 629 g ofcrystals of 4,4′-dihydroxydiphenylsulfone were obtained. The purity ofthe obtained crystals was 95% by weight. By the filtration, 1,767 g of afiltrate was obtained. The ratio of the amounts by weight of4,4′-dihydroxydiphenylsulfone to 2,4′-dihydroxydiphenylsulfone was35:65.

After 865 g of phenol was removed from the filtrate under a reducedpressure at 120° C. or lower, 515 g of water was added, and the ratio ofthe amounts by weight of phenol to water was adjusted at 30:70. Theresultant mixture was heated at 90° C. so that the entire fluid becamehomogeneous. The temperature of the mixture in the homogeneous conditionwas slowly lowered to 30° C., and crystals of2,4′-dihydroxydiphenylsulfone were formed. The formed crystals wereseparated by filtration, washed with water and dried, and 274 g ofcrystals of 2,4′-dihydroxydiphenylsulfone were obtained. The purity ofthe obtained crystals was 85% by weight. A filtrate in an amount of1,034 g was obtained by the filtration.

Example 2

The filtrate obtained by separation of crystals of2,4′-dihydroxydiphenylsulfone by filtration in Example 1 in an amount of1,034 g was concentrated by removing 497 g of a mixture of water andphenol by distillation under a reduced pressure at 120° C. or lower. Theconcentration of phenol in the distillate was 12% by weight. The amountof the concentrated filtrate was 538 g, and the contents of non-volatilecomponents in the concentrated filtrate were as follows:4,4′-dihydroxydiphenylsulfone: 25% by weight;2,4′-dihydroxydiphenylsulfone: 13% by weight; phenolsulfonic acid: 25%by weight; phenol: 25% by weight; and other components: 12% by weight.

Into a reactor, 359 g of the concentrated filtrate obtained above, 1,200g of phenol, 432 g of sulfuric acid and 16 g of phosphonic acid wereplaced, and the dehydration was conducted under a reduced pressure of74.6 to 34.7 kPa at 150 to 165° C. for 6 hours. The amount of thedistillate of a mixture composed of phenol and water during the reactionwas 547 g.

Into the reactor, 267 g of phenol was added, and the dehydration wasconducted under a reduced pressure of 34.7 to 13.3 kPa for 2 hours. Theamount of the entire distillate was 671 g. Then, 267 g of phenol wasadded into the reactor, and the dehydration was conducted under areduced pressure of 34.7 to 13.3 kPa for 2 hours. The amount of theentire distillate was 810 g. Then, 267 g of phenol was added into thereactor, and, after the dehydration was conducted under a reducedpressure of 34.7 to 13.3 kPa for 2 hours, the reaction was completed.The amount of the entire distillate was 960 g.

The amount of the obtained reaction mixture was 1,846 g, and thecontents of the non-volatile components in the obtained reaction mixturewere as follows: 4,4′-dihydroxydiphenylsulfone: 43% by weight;2,4′-dihydroxydiphenylsulfone: 19% by weight; phenolsulfonic acid: 8% byweight; phenol: 27% by weight; and other components: 3% by weight.

The reaction mixture obtained by the dehydration in an amount of 1,846 gwas treated in accordance with the same procedures as those conducted inExample 1, and 601 g of crystals of 4,4′-dihydroxydiphenylsulfone havinga purity of 95% by weight and 223 g of crystals of2,4′-dihydroxydiphenylsulfone having a purity of 90% by weight wereobtained.

Example 3

The same procedures as those conducted in Example 1 were conducted, andcrystals of 4,4′-dihydroxydiphenylsulfone were separated by filtration.A filtrate containing 4,4′-dihydroxydiphenylsulfone and2,4′-dihydroxydiphenylsulfone in amounts such that the ratio of theamount by weight was 35:65 was obtained in an amount of 1,758 g.

After 718 g of phenol was removed by distillation under a reducedpressure at 120° C. or lower from the obtained filtrate, 368 g of waterwas added to adjust the composition of the solvent in the mixture sothat the ratio of the amounts by weight of phenol to water was 50:50.The resultant mixture was heated at 90° C. so that the entire fluidbecame homogeneous. The temperature of the mixture in the homogeneouscondition was slowly lowered to 30° C., and crystals of2,4′-dihydroxydiphenylsulfone were formed. The formed crystals wereseparated by filtration, washed with water and dried, and 262 g of2,4′-dihydroxydiphenylsulfone was obtained. The purity of the obtainedcrystals was 86% by weight.

INDUSTRIAL APPLICABILITY

In accordance with the process of the present invention, highly pure2,4′-dihydroxydiphenylsulfone can be obtained from a mixture of4,4′-dihydroxydiphenylsulfone and 2,4′-dihydroxydiphenylsulfone easilywith a great yield without using organic solvents or a great amount ofan acid or an alkali. Unreacted phenolsulfonic acid contained in thefiltrate obtained by filtration of 2,4′-dihydroxydiphenylsulfone can beused as a portion of the raw materials in the next reaction, and theamount of the raw materials in the production of2,4′-dihydroxydiphenylsulfone can be decreased.

1. A process for producing 2,4′-dihydroxydiphenylsulfone which comprisesseparating 4,4′-dihydroxydiphenylsulfone by crystallization from amixture comprising 4,4′-dihydroxydiphenylsulfone,2,4′-dihydroxydiphenylsulfone, phenolsulfonic acid and phenol which isobtained by dehydration of phenol and sulfuric acid or phenolsulfonicacid so that a mixture having a content of 2,4′-dihydroxydiphenylsulfonegreater than a content of 4,4′-dihydroxydiphenylsulfone is obtained,crystallizing 2,4′-dihydroxydiphenylsulfone by adjusting a compositionof a solvent of an obtained mixture so that a ratio of an amount byweight of phenol to an amount by weight of water is in a range of 10:90to 90:10, and separating 2,4′-dihydroxydiphenylsulfone by filtration. 2.A process for producing 2,4′-dihydroxydiphenylsulfone according to claim1, which comprises adding phenol and sulfuric acid to a filtrateobtained by the separation of 2,4′-dihydroxydiphenylsulfone byfiltration, conducting dehydration using a resultant mixture, separating4,4′-dihydroxydiphenylsulfone by crystallization from a product ofdehydration comprising 4,4′-dihydroxydiphenylsulfone,2,4′-dihydroxydiphenylsulfone, phenolsulfonic acid and phenol so that amixture having a content of 2,4′-dihydroxydiphenylsulfone greater than acontent of 4,4′-dihydroxydiphenylsulfone is obtained, crystallizing2,4′-dihydroxydiphenylsulfone by adjusting a composition of a solvent ofan obtained mixture so that a ratio of an amount by weight of phenol toan amount by weight of water is in a range of 10:90 to 90:10, andseparating 2,4′-dihydroxydiphenylsulfone by filtration.